Phospholipases are involved in the signaling pathway in which a cellular response such as proliferation or secretion is produced consequent to an extracellular stimulus. Activation of mammalian phosphoinositide-specific Phospholipase C (PLC) by a receptor-linked G-protein results in the hydrolysis of phosphatidylinositol-4,5-bisphosphate (PIP2) to release the second messengers 1,2-diacylglycerol (DAG) and 1,4,5-inositol trisphosphate (IP3). DAG activates protein kinase C (PKC), and IP3 releases calcium from stores in the endoplasmic reticulum. Sustained response to the stimulus arises from processing of phosphatidylcholine (PC) by either PLC, which generates DAG directly, or by PLC, which gives phosphatidic acid (PA); PA is then hydrolyzed to DAG.
Phospholipase A2 (PLA2) represents a class of heat-stable, calcium-dependent enzymes catalyzing the hydrolysis of the 2-acyl bond of 3-n-phosphoglycerides. This enzyme is named Phospholipase A2 to denote its 2-acyl specificity (Uthe 1971). Each protein is composed of dimeric subunits, .alpha., .beta., respectively (Wells 1971). PLA2s hydrolyze the sn-2 ester bond on L-glycerophospholipids. At present there are three commonly recognized categories of PLA2s: a large (85 kDa) Ca++ dependent cytosolic PLA2 (Leslie et al: Biochem Biophys. Acta 963:476-492 (1988); Clark et al.: Proc. Natl. Acad. Sci. USA 87:7708-7712 (1990); Kramer et al. 1991), a smaller (40 kDa) Ca++ independent PLA2 found in myocardium (Hazen et al.: J. Biol. Chem. 265:10622-10630 (1990)), and a less well defined group of secreted enzymes found in platelets, synovial fluid, and in some insect and lizard venoms (Verheij et al.: Rev. Physiol. Biochem. Phramacol. 91:91-103 (1981); Dennis: The Enzymes 16:307-353 (1983); Kramer et al. J. Biol. Chem. 264:5768-5775 (1989)). BV-PLA2 is one of many PLA2s in it's class of small, secreted, Ca++ dependent PLA2 enzymes (Dawson: Chemical Studies of Structural Features in Staphylococcus Nuclease T', in Form and Function of Phospholipids, (Ansell, Hawthorne and Dawson eds.), Elsevier, Amsterdam, 97 (1973)).
The first cystolic forms of PLA2 have been implicated in the release of arachidonic acid from cell membranes. This release in arachidonic acid has been reported to be indirectly involved in the inflammatory response. Several types of PLA2s such as Secretory Phospholipase A.sub.2, Phospholipase A.sub.2, and Human Extracellular Group II Phospholipase A.sub.2 are involved in inflammatory diseases such as Rheumatic disease, acute pancreatitis, and skin inflammation, respectively. Arachidonic acid in mast cells, macrophages, monocytes, eosinophils and basophils is released from membrane phospholipids by the activation of phospholipase A2. After its release, arachidonic acid undergoes metabolism via two major pathways: the cyclooxygenase pathway (which produces various prostaglandins and thromboxanes) and the 5-lipoxygenase pathway (which produces leukotrienes). Leukotrienes are "slow reacting substances of anaphylaxis" and have been named A, B, C, D, and E leukotrienes and these leukotrienes subtypes play a crucial role in asthma.
Phospholipases are targets of a variety of body system modulating agents one of which is endothelin, an endothelium derived vasoactive polypeptide which is the most potent vasoconstrictor identified to date. This protein modulates vascular smooth muscle tone as well as participating in the long term control of the cell cycle involved in the chronic remodeling of the vascular tree. Additionally, endothelin interacts with phospholipases via guanine nucleotide regulatory proteins using a common guanine nucleotide modulating machinery. Thus, phospholipases are an important component of the complex array of effectors which regulate the response of organs, such as the heart, to disease states and injury recovery.
Given the importance of such phospholipases in the regulation of lipid metabolism, signal transduction, and cell cycle control, there exists a need to identify novel phospholipases which function as modulators in these processes such as the suppression of inflammation and oncogenesis and whose aberrant function can result in disorders arising from improper signal transduction such as cancer, inappropriate levels of phospholipase metabolites which mediate the anaphalactic response in respiratory disorders such as asthma, as well as improper responses to disease states such as chronic heart failure which direct tissues into a remodeling paradigm. Further, phospholipases may be potential drug target candidates in a variety of disease areas, including anticancer drugs, cardiovascular drugs, and anti-inflammatory drugs.